Temperature, pressure, and composition of hydrothermal fluids, with their bearing on the magnitude of tectonic uplift at mid‐ocean ridges, inferred from fluid inclusions in oceanic layer 3 rocks

Abstract

Quartz‐bearing veins in metagabbroic rocks dredged from the Mathematician Ridge, east Pacific, contain abundant fluid inclusions. Heating and freezing data on nearly 400 inclusions from seven samples allow determination of the temperatures, pressures, and fluid compositions in the subseafloor hydrothermal system at the time of quartz growth. Coexisting dense halite‐saturated inclusions and low‐density, low‐salinity vapor‐rich inclusions (average 45 and 2 wt % NaCl equivalent, respectively) attest to an episode of phase separation in some samples. The phase separation occurred at temperatures of about 600°–700°C and pressures of 60–100 MPa (600–1000 bars). The fact that samples that formed at 60–100 MPa are now exposed on the seafloor, where ambient hydrostatic pressure is only 30–35 MPa, suggests that the samples have been tectonically uplifted of the order of 3 km. The fluids could originally have been part of a deep axial hydrothermal circulation cell, or alternatively, they could have been formed in a deep convection cell underlying the off‐axis edges of a magma chamber. Fluids are NaCl‐CaCl 2 brines with molar Na: Ca of 4–8. This range of molar Na: Ca is very close to that of the inferred hydrothermal end‐member from various active black smokers, to the measured ratios from basalt‐seawater interaction experiments, and to the ratio calculated during numerical basalt‐seawater interaction calculations. Crushing experiments indicate little or no compressible gas within the fluids. Fluid inclusions in albite suggest trapping temperatures of around 410°–500°C. Those in epidote may have been trapped at around 500°C and 110 MPa (1.1 kbar) pressure, or around 3 km beneath the Mathematician Ridge seafloor.